For a liquid-crystal display element to function, the direction of orientation of liquid-crystal molecules and the direction of polarization of the polarizer must be set in a particular relation to each other. In liquid-crystal display element technologies, LCDs using a TN (Twisted Nematic) type liquid-crystal were the first to be put into practical use. Recently, LCDs using a VA (Vertical Alignment) type liquid-crystal, an IPS (Inplane Switching) type liquid-crystal etc. were put into practical use. Although a technical explanation is omitted, in an LCD using such TN-type liquid-crystal panel, liquid-crystal molecules are provided between two upper and lower orientation films having respective rubbing directions on the inner surfaces of the substrates of the liquid-crystal panel. This means that the liquid-crystal molecules are twisted by 90 degrees along the optical axis so that, when a voltage is applied, the liquid-crystal molecules are aligned in a direction perpendicular to the orientation films. However, in the case where the LCD is designed to allow images of the same quality to be seen from right and left sides of a display screen as those view from directly in front of the display screen, the direction of rubbing on the orientation film at the viewing-side must be 45 degrees (the rubbing direction of the other orientation film being 135 degrees). It is therefore necessary that the polarizing sheets made from the polarizing composite films as shown in FIGS. 1A and 1B, be laminated respectively on the front and back sides of the liquid-crystal panel with polarizers respectively oriented in directions inclined respectively by 45 degree with respect to a lengthwise or widthwise direction of the display screen so as to conform to the rubbing directions.
Therefore, in a polarizing sheet for use in producing a liquid-crystal element of a TN-type liquid-crystal panel, it is required that the optical film is punched-out or cut into a rectangular-shaped sheet having a long side or a short side determined in accordance with the size of the TN liquid-crystal panel, and inclined by 45 degrees with respect to the orientation direction of the polarizer produced by stretching in the lengthwise or widthwise direction. This is described in Japanese Laid-Open Patent Publication No. JP 2003-161935A or Japanese Patent No. 3616866B. The width of the sheet to be processed into the rectangular shape, that is, the short side of the sheet, is smaller than the width of the continuous web.
The punching or cutting the optical film sheet into the rectangular-shaped sheet from the continuous web of an optical film may be collectively referred to as “individualized sheets” or “a method and system for manufacturing individualized sheets” for liquid-crystal display elements. The optical film sheet thus punched-out or cut is produced by punching or cutting not only the surface protection film contained in the optical film but also the carrier film protecting the exposed surface of the adhesive layer in the polarizing composite film together. The integrally punched-out or cut carrier film sheet may be referred to as “separator”, rather than “carrier film sheet”. Thus, the manufacturing process of the liquid-crystal display elements includes the first step of peeling the separator from each of optical film sheet to have the adhesive layer of the polarizing sheet exposed. Subsequently, the optical film sheet each having the adhesive layer exposed are conveyed one-by-one by for example under a vacuum suction irrespective of whether the surface protective film sheets are laminated or not, and laminated to respective ones of a plurality of liquid-crystal panels. According to the abovementioned manufacturing process of the liquid-crystal display elements, it has been required that the integrally punched-out or cut sheet from the continuous web of optical film is in the form of an individualized sheet having four trimmed sides and a certain level of stiffness of less deflection or bend and which can be conveyed and laminated easily. During the initial period in the history of the manufacturing process of the liquid-crystal display elements, this optical film sheet or the polarizing sheet contained in the optical film sheet was generally known as a “polarizing plate” which is still used as a common name.
In the manufacturing process of TN-type liquid-crystal display elements, an optical film fed out from a roll of the optical film laminate is integrally and sequentially punched-out or cut in a direction transverse to the feed direction. However, in this case, it is impossible to obtain a finished liquid crystal display element simply by sequentially laminating the sheets formed to respective ones of a plurality of liquid-crystal panels. This is because the sheets each formed with a long or short side extending in a direction 45 degrees cannot be laminated sequentially to respective ones of the liquid-crystal panels W with the same posture. Therefore, to provide a finished liquid-crystal display element by transporting a polarizing sheet to a position for lamination with a liquid-crystal panel, and then laminating the polarizing sheet to the liquid-crystal panel, an optical film having a width greater than a long side of a liquid-crystal panel is fed out in a lengthwise direction, and punched-out at an angled direction of 45 degrees with respect to the lengthwise direction for each of the optical film, using for example a die, into a plurality of individual sheets, and appropriately fed to the laminating process of the liquid-crystal panel as shown in Japanese Laid-Open Patent Publication No. JP 2003-161935A or Japanese Patent 3616866 B. Alternatively, an optical film having a substantial longitudinal length may be provided by preparing a continuous web of optical film having a substantially large width and punching or cutting the web at an angle of 45 degrees with respect to the longitudinal direction to provide a film sheet which can be used as an optical film having a substantial length, or may be provided by connecting together a plurality of such obliquely cut sheets of the optical film, as shown in Japanese Patent Publication No. 62-14810 B, and the optical film as produced in such process of forming sheets from an optical film having the width of the liquid-crystal panel is then continuously fed and cut in the widthwise direction with respect to its feeding direction to provide a plurality of sheet strips each having a required length and each including a plurality of polarizing sheet, the plurality of polarizing sheets in the sheet strip being then laminated to respective ones of a plurality of liquid-crystal panels sequentially conveyed to provide completed liquid-crystal display elements. At any rate, the above techniques are not beyond the system for manufacturing individualized sheets.
Japanese Patent Publication No. 62-14810 B was published before the VA-type liquid-crystal and the IPS-type liquid-crystal are brought into practical use and discloses an apparatus to produce a liquid-crystal panel by sequentially laminating a plurality of sheets formed into a required length onto respective ones of a plurality of liquid-crystal panels while continuously feeding an optical film containing a polarizing composite film. Japanese Patent Publication No. 62-14810 B discloses a technique of continuously feeding an optical film which comprises a polarizing composite film (called “elongated polarizing plate”) and a separator for protecting an adhesive layer on the polarizing composite film, “cutting only a polarizing plate 4 and an adhesive layer while leaving a separator uncut (hereinafter referred to as “half-cut”)”, removing defective polarizing sheets formed in the course of the feeding, sequentially laminating the peeled sheets to the liquid-crystal panels (called “liquid-crystal cells”) for constituting small-size display screens of an electronic calculators or the like, while peeling the separator from the polarizing sheets. The apparatus is a labeler unit which produces an LCD using a TN-type liquid-crystal. Thus, the optical film to be used must be an elongated sheet produced from an optical film cut it in a direction 45 degrees oblique to the longitudinal direction of the optical film with a width corresponding to the liquid-crystal panel. Therefore, this apparatus cannot be applied directly to a manufacturing apparatus adapted to perform steps of continuously forming a plurality of polarizing sheets from an optical film having a laminated structure and laminating respective sheets to respective ones of the liquid-crystal panel using VA-type liquid-crystal and the IPS-type liquid-crystal to make a liquid-crystal display element because of the width of optical film required.
Automation of manufacturing process of liquid-crystal display elements using individualized sheets is generally described below. For example, in Japanese Laid-Open Patent Publication No. 2002-23151A. Flexible individualized sheets tend to be bowed or warped by being bent or distorted at its edge portions, and such tendencies have caused a serious technical impediment to accuracy and speed in registration and lamination with liquid-crystal panels. Thus, it will be understood that the individualized sheet is required to have a certain level of thickness and stiffness to facilitate transport under suction and lamination to the liquid-crystal panel. For example, Japanese Laid-Open Patent Publication No. 2004-144913A, Japanese Laid-Open Patent Publication No. 2005-298208A or Japanese Laid-Open Patent Publication No. 2006-58411A disclose measures for addressing such technical problems.
On the other hand, the VA-type and IPS-type liquid-crystal panels are not designed to have a twisted structure of liquid-crystal molecules. Thus, in producing liquid-crystal display elements using these types of liquid-crystal panels, it is no longer required to have the polarization axis of the polarizing sheet oriented 45 degrees. In the case of liquid-crystal display elements using these types of liquid-crystal panels, the liquid-crystal display element is formed by applying the polarizing sheets to the opposite sides of the liquid-crystal display panel oriented with their polarization axes crossed at 90 degrees each other. In the case of the VA-type and IPS-type liquid-crystal panels, with respect to the viewing angle characteristics, maximum contrast can be obtained along the direction of the polarizing axis of the polarizing sheet, so that it is preferable that the sheets have optical axes oriented in parallel with the longitudinal or transverse direction of the liquid-crystal panel from the technical view point of symmetry of the viewing angle characteristics and visibility. Thus, these sheets to be applied to the liquid-crystal panel have a feature that the optical film including a polarizing composite film which has been subjected to longitudinal or transverse stretching can be continuously fed out from a roll and cut along transverse lines with respect to the feed direction of the optical film to sequentially produce rectangular sheets including the polarizing sheets having same width as the optical film width.
Because of the improved viewing angle characteristics, VA-type or IPS-type liquid-crystal are more widely adopted than the TN-type. In view of such trend in environments of technical developments, proposals have been made such as the one described in Japanese Laid-Open Patent Publication No. 2004-361741A which is a technique for enhancing manufacturing efficiency based on use of the VA-type or IPS-type liquid-crystal panels and comprises steps of continuously feeding an optical film, cutting the optical film in conformity to the size of a liquid-crystal panel and sequentially laminating a plurality of cut rectangular sheets comprising the polarizing sheets which have been produced by the cutting step to respective ones of a plurality of the liquid-crystal panels.
However, the mainstream of manufacture of liquid-crystal display elements is still based on manufacturing technology utilizing individualized sheets, due to the following technical problems. In manufacturing liquid-crystal display elements, a critical technical challenge is to detect any defect which may otherwise be retained in the display elements to be formed, and to prevent any defective product from being produced. This makes it possible to significantly improve manufacturing yield. Most of the product defects primarily arise from defects in the polarizing composite film contained in the optical film. However, it is not practical to provide an optical film after completely removing all defects contained in individual films before they are laminated together to form the optical film. The reason is that, observation of the polarizer, protective film laminated on the polarizer and an adhesive layer formed on the polarizing composite film indicates that there are various kinds of defects, including defects inherent in the PVA film of the polarizer itself, defects arose in connection with the lamination of the protective film to the polarizer or defects generated in the adhesive layer of the formed polarizing composite film, distributed in 20 to 200 positions over a unit length of the polarizing composite film of 1000 m. This means that under existing circumstances, it is extremely difficult to produce a defect-free optical film. It is not permitted to use an optical film sheet having visible flaws or defects as a sheet for television even if such a flaw or defect is small. Therefore, if lengths of the polarizing composite film with defects are used to form a display and a display requires 1 m of film, 20 to 200 defective liquid-crystal display elements out of 1,000 products will be produced.
A proposed preliminary inspection apparatus for a polarizing composite film, is disclosed, for example, in Japanese Patent No. 3974400B, Japanese Laid-Open Patent Publication Nos. 2005-62165A and 2007-64989A.
Japanese Laid-Open Patent Publication 2007-140046A discloses a method wherein the method comprises peeling a carrier film (called “releasable film”) from an optical film (called “polarizing plate stock”) fed out continuously from a roll of an optical film laminate to expose a polarizing composite film (called “polarizing plate”) having an adhesive layer; detecting a defect or defects present in the polarizing composite film; punching or cutting only normal regions of a polarizing composite film into a rectangular shape, while leaving the defective region or regions of the polarizing composite film untouched. Japanese Patent Application No. 2007-266200 discloses an invention relating to a method and system for laminating a polarizing film sheet to a liquid-crystal panel. The method and an apparatus disclosed in Japanese Patent Application No. 2007-266200, however, require that steps cause not only substantial complexity in the entire system for laminating but also an increase in the number of steps and difficulty in control for each step, and therefore, cause corresponding reduction in the manufacturing speed.
The present disclosure has been made based on the above related disclosures and through intensive researches and considerations for significantly enhancing product accuracy and manufacturing speed, and drastically improving production yield, in the manufacture of liquid-crystal display elements.